The present invention relates to doped carbon nanotubes and more particularly, to transparent conductive films formed of the doped carbon nanotubes.
Transparent conducting electrodes are key components of many modern electronic devices including photovoltaic cells, organic light emitting diodes, flat panel displays and touch screens. The most widely used transparent conducting electrode is indium tin oxide (ITO). Despite the exceptional optoelectronic properties of ITO (e.g., sheet resistance of 5 to 10 ohms per square (Ω/□) at >85% transmittance), the material suffers from considerable drawbacks including increased materials cost due to scarcity of indium and the costs associated with high temperature vacuum deposition. Additionally, vacuum deposited ITO films are brittle and therefore not suitable for flexible electronics.
It turns out that very thin carbon nanotube films as thin as 10 or 20 nanometers are transparent to visible light and can conduct electricity, which makes them candidates for transparent conducting electrodes. Doping of the carbon nanotubes in films can increase overall conductivity. Currently, the most commonly used chemical dopants for nanotube networks are nitric acid and thionyl chloride. Although these materials provide excellent doping efficiency, these materials also have drawbacks. For example, both chemicals are relatively harsh chemicals and require special handling. Still further, the volatility of these materials leads to sheet resistances that are unstable over time and increases to values approaching those of undoped films.
To address these drawbacks, single electron oxidants have been developed, which can utilize milder solvents. These single electron oxidants dope the nanotubes by removing an electron from the carbon nanotube, presumably forming a stable charge transfer complex. However, these materials have exhibited low doping efficiency. For example, acid dopants have been found to provide 50% more doping efficiency than the prior art single electron oxidant dopants. Moreover, current single electron oxidant dopants do not exhibit good stability. Although the charge transfer complex should have resulted in greater stability, this has not been observed experimentally.
Accordingly, there is a need for alternative materials for use as transparent conducting electrodes, especially materials that avoid the use of nitric acid and thionyl chloride as chemical dopants, are non-volatile, and have a sheet resistance and an optical transmittance suitable for use as transparent conducting electrodes.